The present invention relates generally to devices for machining or rebuilding internal combustion engines and, more specifically, to devices for boring engine overhead camshaft cylinder heads.
The cylinder heads of overhead cam engines have bearings for supporting the camshaft. Each bearing is located in a tower that positions the camshaft relative to the cylinder head. The most commonly used type of bearing consists only of the interior surface of the tower. Typically, between two and seven bearings and corresponding towers are distributed along the length of the camshaft in the cylinder head. Each tower comprises a portion that is formed integrally with the remainder of the cylinder head. In a few types of cylinder heads, the entire tower is integrally formed with the cylinder head. Such a tower completely encircles the camshaft with the inner surface of the tower forming the bearing. However, in most types of cylinder heads, the tower is in two sections, the base portion of which is formed integrally with the cylinder head. The camshaft is supported between a semicircular bearing surface in the base portion and a corresponding semicircular bearing surface in the cap. The cap is secured to the base portion using two bolts.
The camshaft rotates smoothly so long as the bearings remain aligned along the camshaft axis of rotation. The cylinder head may, however, warp as a result of engine overheating. In every case, this warpage results in a concave deformation of the cylinder head. In addition, the bearings may wear over time as a result of use. Both cylinder head warping and bearing wear may cause the camshaft to vibrate and ultimately may prevent the camshaft from turning at all, or the camshaft bearings may wear so quickly and severely that the oil pressure drops, causing engine failure. Thus, it is apparent that when cylinder head warpage and bearing wear occurs, the camshaft bearings must be repaired in order to avoid costly repairs or engine replacement.
A line boring machine is a device having a table, a rotating steel boring spindle or boring bar, and a motor connected to the bar. The cylinder head is secured to the table, which functions as a reference plane. The boring bar is received horizontally through all the cylinder head bearings. The boring bar has mounting recesses distributed along its length for receiving cutting bits. In conventional boring bars, the mounting recesses are arranged along a common line parallel to the bar's axis. One bit is mounted adjacent each tower. The machine includes drive mechanisms for rotating the bar and moving the bar longitudinally along its axis of rotation. The bar is simultaneously rotated and fed longitudinally. Each cutting bit engages a bearing and removes metal to enlarge the bearing diameter. The cylinder head may then be removed from the machine. In order to provide the proper bearing diameter to meet OEM specifications, "repair bearings," which are annular inserts, usually made of steel, having an inside diameter equal to the proper diameter for the camshaft bearings and an outside diameter approximately equal to the diameter of the newly enlarged bearing, are inserted into the enlarged bearings and are retained by the resulting friction-fit. The camshaft may then be re-inserted through the repair bearings.
The use of repair bearings has several disadvantages. The friction-fit holding the repair bearings may loosen, allowing the repair bearings to rotate with respect to the cylinder head. Such rotation will quickly result in engine failure and require further repairs. In addition, heat conduction between the cylinder head, which is typically aluminum, and the steel repair bearings is poor and may prevent heat generated by the camshaft friction from dissipating properly into the cylinder head. The non-uniform heat distribution and the different coefficients of thermal expansion of the two metals increase the risk of loss of adhesion between the repair bearings and the cylinder head.
The use of the line boring machine described above to repair camshaft bearings creates a problem. The boring bar and its cutting bits must remain precisely axially aligned with the bearings during the process. In prior art line boring machines, the boring bar must be supported because the effect of gravity on the horizontal bar tends to sag or bow downward, thereby preventing it from boring along a perfectly straight axis.
Line boring machines attempt to minimize this problem by supporting the bar at multiple points along its length. The line boring machine includes multiple support arms that have bearings in which the bar rotates. When a cylinder head is mounted on the table of the machine, the arms extend between the towers. If the towers are spaced closely together, however, as is common in small engines, insufficient space exists between the towers to accommodate an arm. Moreover, both the distance between the arms and the distance between each arm and the table can be adjusted. It is therefore both time-consuming and difficult to obtain the required alignment among all of the arms.
Another solution that has been attempted involves supporting the boring bar by the two camshaft bearings at the extreme ends of the cylinder head. A bearing ring is inserted into each end bearing, and the boring bar is inserted through the bearing rings. This method is not effective, however, if the end bearings are themselves in poor alignment with each other. When this method is used, the end bearings tend to wear more quickly than the other bearings. Furthermore, the effectiveness of the method decreases with increasing cylinder head length.
These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.